The invention relates to a variable stroke valve drive for an internal combustion engine having an intermediate lower and a crank arm and cam arrangement for actuating the lever.
In German Unexamined Patent DE 101 23 186 A1, in which a mechanical regulating means for adjusting the lift of a gas exchange valve, a variable stroke valve drive of an internal combustion engine is described. The mechanical regulating means is characterized in that the rotational speed control and load control of the internal combustion engine are regulated not via a throttle valve but instead via the valve lift of the gas exchange intake valves. To achieve this, the mechanical regulating means has an intermediate lever which is mounted at one end so it is slidingly movable on a crank path of a crank arm and has a working cam with a null lifting cam and a lifting cam at the other end. The working cam is operatively connected here to a gas exchange valve via an intermediate element, namely a drag lever. Furthermore, the mechanical regulating means has a camshaft with which the intermediate lever is rotated against the elasticity of the restoring spring about a point near the crank arm, so that the portions between the null lifting cam and the lifting cam, which are operatively connected to the intermediate element, are shifted. Despite the rotating camshaft, the gas exchange valve remains closed as long as the contact point and/or the contact line is between the intermediate lever or the null lifting cam and a rolling element arranged on this intermediate lever. Furthermore, the mechanical regulating means has a second adjusting element, namely an eccentric shaft, which acts on the intermediate lever near the crank arm. Due to the rotation of the eccentric shaft, the intermediate lever in the crank arm is shifted parallel to the crank path so that the effective component between the lifting cam and the no-load lifting cam is altered. The portion of the lifting cam with respect to the cam elevation may thus be increased or decreased. An increase in the lifting cam portion corresponds to an increase in the gas exchange valve lift. A reduction in the lifting cam portion corresponds to a reduction in the gas exchange valve lift until as described above, only the no-load lifting cam portion is operatively connected to the intermediate lever.
One disadvantage of the embodiment described here is the relatively high wear on the intermediate lever on all contact areas with corresponding friction partners such as the restoring spring, the crank arm, and the drag lever.
The object of the present invention is to provide a generic variable stroke valve drive with minimized wear.
This object is achieved through the features in the by using instead of a camshaft, a cam plate whose radius increases or decreases steadily over the circumference with respect to the axis of rotation.
In the state of the art described above, the camshaft has a base circle, i.e., a circumferential area of the cam with a constant radius. As long as the base circle of the camshaft is operatively connected to the intermediate lever, the intermediate lever remains at rest, i.e., it is not rotated. Due to the fact that it is stationary, there is a transition from adhesion to sliding on the contact areas of the intermediate lever with a spring element, the intermediate element and the crank arm in the transition from the base circle of the cam to the cam elevation, thus resulting in heavy wear on the contact areas.
With an embodiment of the present invention, however, the intermediate lever is kept permanently in motion with rotation of the inventive cam plate. Due to the constant motion of the intermediate lever, tangential excitation of the spring element due to breakaway is prevented, while avoiding the high acceleration forces that are transmitted via the contact points, at which unfavorable lubrication conditions prevail due to a static surface pressure. In other words, through the proposed constant vibrational movement of the intermediate lever when using a cam plate in an advantageous manner is there a constant oil input between the contact areas of the contact partners, consisting of the intermediate lever, the spring element and the intermediate element. Thus the fictional losses and the component wear are greatly reduced and the lifetime of the variable stroke valve drive is greatly prolonged. Secondly, the intermediate lever rotational accelerations due to the uninterrupted rotational movement are greatly reduced, so the gas exchange valves can be opened more quickly and the charge cycle, as well as processing of the mixture, are improved. Thirdly, resonance effects of the restoring spring are ruled out due to constant active leg lengths, such as those which occur with a stationary intermediate lever in contact with the base circle, and the variable stroke valve drive is more stable mechanically, i.e., is less susceptible from the standpoint of vibration technology. Furthermore, as a result of this, the spring element may be designed with smaller dimensions, so that much higher rotational speeds can also be achieved in conjunction with the reduced acceleration forces of the intermediate lever, as described previously.
Due to the inventive use of the cam plate, the variable stroke valve drive thus becomes much more resistant to wear and more stable mechanically, i.e., there are reduced acceleration forces and vibrational forces, thereby reducing technical vibration problems and allowing the rotational speed of the internal combustion engine to be increased with no problem.
Through support of the intermediate lever on the crank path via a roller element, the internal friction in the entire variable stroke valve drive is reduced again significantly. Due to the proposed embodiment, wear is thus further reduced and the lifetime and/or service life is increased. Fuel savings due to the reduced internal friction of the variable stroke valve drive can be mentioned as another positive effect.
Further, through arrangement of the crank path as an arc of a circle, a purely rotational movement of the intermediate lever is possible in operation of the second adjusting device. The fulcrum here is the point near the crank arm and thus when using the first roller element this is the axis of rotation of the first roller element. There are no translational movements and thus sliding movements, which are associated with wear. Furthermore, spontaneous (i.e., without delay) opening and closing of the gas exchange valve are possible.
With an embodiment in which the base circle of a camshaft is simulated, a closed gas exchange valve is made possible without resulting in the aforementioned disadvantages of a traditional known camshaft.
An embodiment in which the ramp between the null lifting cam and the lifting cam is integrally molded reduces the acceleration forces that occur in the variable stroke valve drive in the transition from the no-load lifting cam to the lifting cam. The resulting constant opening and closing accelerations of the intermediate lever allow a higher rotational speed of the internal combustion engine.
With an intermediate element configured as a swing lever or a tilt lever, the variable stroke valve drive is largely free of play and maintenance. A hydraulic valve play equalizing element is preferably used.
An embodiment in which the inventive variable stroke valve drive crank arm is located in a cylinder head allows a compact and stiff design of the variable stroke valve drive.
Using a second adjusting device, the forces and/or torques to be applied in adjusting the gas exchange valve lift can be achieved with no problem. Of course the cam plate may have any technically feasible contour.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.